Another aspect of perception that is common to both realists and anti-realists is the idea of mental or perceptual space. [[David Hume]] considers this at some length and concludes that things appear extended because they have the attributes of colour and solidity. A popular modern philosophical view is that the brain cannot contain images so our sense of space must be due to the actual space occupied by physical things. However, as [[Rene Descartes]] noticed, perceptual space has a projective geometry, things within it appear as if they are viewed from a point and are not simply objects arranged in 3D. Mathematicians now know of many types of projective geometry such as complex [[Minkowski space]] that might describe the layout of things in perception (see Peters (2000)). It is also known that many parts of the brain contain patterns of electrical activity that correspond closely to the layout of the retinal image (this is known as [[retinotopy]]). There are indeed images in the brain but how or whether these become conscious experience is a mystery (see McGinn (1995)).

Another aspect of perception that is common to both realists and anti-realists is the idea of mental or perceptual space. [[David Hume]] considers this at some length and concludes that things appear extended because they have the attributes of colour and solidity. A popular modern philosophical view is that the brain cannot contain images so our sense of space must be due to the actual space occupied by physical things. However, as [[Rene Descartes]] noticed, perceptual space has a projective geometry, things within it appear as if they are viewed from a point and are not simply objects arranged in 3D. Mathematicians now know of many types of projective geometry such as complex [[Minkowski space]] that might describe the layout of things in perception (see Peters (2000)). It is also known that many parts of the brain contain patterns of electrical activity that correspond closely to the layout of the retinal image (this is known as [[retinotopy]]). There are indeed images in the brain but how or whether these become conscious experience is a mystery (see McGinn (1995)).

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Our perception of the external world begins with the senses, which lead us to generate empiricalconcepts representing the world around us, within a mental framework relating new concepts to preexisting ones. Because perception leads to an individual's impression of the world, its study may be important for those interested in better understanding communication, self, id, ego — even reality.

While René Descartes concluded that the question "Do I exist?" can only be answered in the affirmative (cogito ergo sum), Freudianpsychology suggests that self-perception is an illusion of the ego, and cannot be trusted to decide what is in fact real. Such questions are continuously reanimated, as each generation grapples with the nature of existence from within the human condition. The questions remain: Do our perceptions allow us to experience the world as it "really is?" Can we ever know another point of view in the way we know our own?

Internal perception (proprioception) tells us what's going on in our bodies. We can sense where our limbs are, whether we're sitting or standing; we can also sense whether we are hungry, or tired, and so forth.

External or Sensory perception (exteroception), tells us about the world outside our bodies. Using our senses of sight, hearing, touch, smell, and taste, we discover colors, sounds, textures, etc. of the world at large. There is a growing body of knowledge of the mechanics of sensory processes in cognitive psychology.

The philosophy of perception is mainly concerned with exteroception. When philosophers use the word perception they usually mean exteroception, and the word is used in that sense everywhere .

The science of perception is concerned with how events are observed and interpreted. An event may be the occurrence of an object at some distance from an observer. According to the scientific account this object will reflect light from the sun in all directions. Some of this reflected light from a particular, unique point on the object will fall all over the corneas of the eyes and the combined cornea/lens system of the eyes will divert the light to two points, one on each retina. The pattern of points of light on each retina forms an image. This process also occurs in the case of silouettes where the pattern of absence of points of light forms an image. The overall effect is to encode position data on a stream of photons and to transfer this encoding onto a pattern on the retinas. The patterns on the retinas are the only optical images found in perception, prior to the retinas light is arranged as a fog of photons going in all directions.

The images on the two retinas are slightly different and the disparity between the electrical outputs from these is resolved either at the level of the lateral geniculate nucleus or in a part of the visual cortex called 'V1'. The resolved data is further processed in the visual cortex where some areas have relatively more specialised functions, for instance area V5 is involved in the modelling of motion and V4 in adding colour. The resulting single image that subjects report as their experience is called a 'percept'. Studies involving rapidly changing scenes show that the percept derives from numerous processes that each involve time delays (see Moutoussis and Zeki (1997)).

Recent fMRI studies show that dreams, imaginings and perceptions of similar things such as faces are accompanied by activity in many of the same areas of brain. It seems that imagery that originates from the senses and internally generated imagery may have a shared ontology at higher levels of cortical processing.

If an object is also a source of sound this is transmitted as pressure waves that are sensed by the cochlear in the ear. If the observer is blindfolded it is difficult to locate the exact source of sound waves, if the blindfold is removed the sound can usually be located at the source. The data from the eyes and the ears is combined to form a 'bound' percept. The problem of how the bound percept is produced is known as the binding problem and is the subject of considerable study. The binding problem is also a question of how different aspects of a single sense (say, color and contour in vision) are bound to the same object when they are processed by spatially different areas of the brain.

The most common theory of perception is naïve realism in which people believe that what they perceive is things in themselves. Children develop this theory as a working hypothesis of how to deal with the world. Many people who have not studied biology carry this theory into adult life and regard their perception to be the world itself rather than a pattern that overlays the form of the world. Thomas Reid took this theory a step further, he realised that sensation was composed of a set of data transfers but declared that these were in some way transparent so that there is a direct connection between perception and the world. This idea is called Direct Realism. Direct Realism has become popular in recent years with the rise of postmodernism and Behaviourism. Direct Realism does not clearly specify the nature of the bit of the world that is an object in perception, especially in cases where the object is something like a silhouette.

The succession of data transfers that are involved in perception suggests that somewhere in the brain there is a final set of activity, called sense data, that is the substrate of the percept. Perception would then be some form of brain activity and somehow the brain would be able to perceive itself. This concept is known as indirect realism. In Indirect Realism it is held that we can only be aware of external objects by being aware of representations of objects. This idea was held by John Locke and Immanuel Kant. The common argument against indirect realism, used by Gilbert Ryle amongst others, is that it implies a homunculus or Ryle's regress where it appears as if the mind is seeing the mind in an endless loop. This argument assumes that perception is entirely due to data transfer and classical information processing. This assumption is highly contentious (see strong AI) and the argument can be avoided by proposing that the percept is a phenomenon that does not depend wholly upon the transfer and rearrangement of data.

Direct realism and indirect realism are known as 'realist' theories of perception because they hold that there is a world external to the mind. Direct realism holds that the representation of an object is located next to, or is even part of, the actual physical object whereas indirect realism holds that the representation of an object is brain activity. Direct realism proposes some as yet unknown direct connection between external representations and the mind whilst indirect realism requires some feature of modern physics to create a phenomenon that avoids infinite regress. Indirect realism is consistent with experiences such as:

binding, dreams, imaginings, hallucinations, illusions, the resolution of binocular rivalry, the resolution of multistable perception, the modelling of motion that allows us to watch TV, the sensations that result from direct brain stimulation, the update of the mental image by saccades of the eyes and the referral of events backwards in time

whereas direct realism argues either that these experiences do not occur or avoids the problem by defining perception as only those experiences that are consistent with direct realism.

Apart from the realist theories of perception there are also anti-realist theories. There are two varieties of anti-realism: Idealism and Skepticism. Idealism holds that we can only be aware of mental things whereas skepticism holds that because we never perceive external objects directly we can never know for certain whether they exist. One of the most influential proponents of idealism was George Berkeley who maintained that everything was mind or dependent upon mind.
Berkeley's idealism has two main strands, phenomenalism in which physical events are viewed as a special kind of mental event and subjective idealism. David Hume is probably the most influential proponent of skepticism.

The philosophy of perception is very closely related to a branch of philosophy known as epistemology, the theory of knowledge, and many of the ideas presented above are also discussed under this heading.

Perception is sometimes referred to as a cognitive process in which information processing is used to transfer information from the world into the brain and mind where it is further processed and related to other information. Some philosophers and psychologists propose that this processing gives rise to particular mental states (cognitivism) whilst others envisage a direct path back into the external world in the form of action (radical behaviourism).

Many eminent behaviourists such as John B. Watson and B.F. Skinner have proposed that perception acts largely as a process between a stimulus and a response but despite this have noted that Ryle's "ghost in the machine" of the brain still seems to exist. As Skinner wrote:

"The objection to inner states is not that they do not exist, but that they are not relevant in a functional analysis" Skinner 1953.

This view, in which experience is thought to be an incidental by-product of information processing, is known as epiphenomenalism.

Another aspect of perception that is common to both realists and anti-realists is the idea of mental or perceptual space. David Hume considers this at some length and concludes that things appear extended because they have the attributes of colour and solidity. A popular modern philosophical view is that the brain cannot contain images so our sense of space must be due to the actual space occupied by physical things. However, as Rene Descartes noticed, perceptual space has a projective geometry, things within it appear as if they are viewed from a point and are not simply objects arranged in 3D. Mathematicians now know of many types of projective geometry such as complex Minkowski space that might describe the layout of things in perception (see Peters (2000)). It is also known that many parts of the brain contain patterns of electrical activity that correspond closely to the layout of the retinal image (this is known as retinotopy). There are indeed images in the brain but how or whether these become conscious experience is a mystery (see McGinn (1995)).